The present invention relates to a method and apparatus for purifying exhaust gases from a diesel engine, more particularly the present invention relates to a method and apparatus for purifying exhaust gases from a diesel engine which is capable of removing particulates and NOx contained in the exhaust gases.
Present regulatory conditions in the automotive market have led to an increasing demand to improve fuel economy and reduce emissions in present vehicles. These regulatory conditions must be balanced with the demands of a consumer for high performance and quick response for a vehicle.
A diesel engine has an efficiency of up to about 52% and is thus the best converter of fossil energy. NOx emission concentration is dependent upon local oxygen atom concentration and the local temperature. Said high efficiency is however only possible at an elevated combustion temperature at which high NOx levels are inevitable. Moreover, a suppression of NOx formation by internal means (air/fuel ratio) has the tendency to cause an increase in particulates, known as the NOx-particulates trade off. Furthermore, an excess of oxygen in the exhaust gas from a diesel engine prevents the use of stoichiometric 3-way-catalyst technology for reduction of NOx as is used in gasoline engine cars from the late 80-ties. Reducing the oxides of nitrogen (NOx) and particulate matter (PM) in exhaust gases from a diesel engine has become a very important problem in view of the protection of environment and the saving of finite fossil energy supply. For the coming legislation (US10, EU V1 etc) it may be necessary to have a combination of a diesel oxygen catalyst (DOC), a diesel particulate filter (DPF) and a selective catalytic reduction (SCR) catalyst in the exhaust system. When designing the DOC and the DPF for a combined system, the noble metal loading is generally a compromise. Good NO-oxidation (i.e. good passive regeneration) and good HC-oxidation (i.e. good O2-regeneration and exhaust after treatment system (EATS) heating) require high noble metal loading. However, a too high noble metal loading (i.e. too high NO-oxidation) will decrease NOx-conversion efficiency in the SCR, and also lead to emissions of the green house gas N2O. When taking into account that the NO- and HC-oxidation must be sufficient also for an aged catalyst, this problem becomes even greater. In the end, if high SCR-activity is needed on the fresh system (for e.g. certification), the aged NO- and HC oxidation will be less than optimal.
Today the catalyst specification is a compromise and the engine parameters are set to fulfil the certification cycle regarding NOx. In JP 2006207512 a DOC bypass (without DPF) is described, which is controlled by the temperature of the DOC. With a bypass solution a higher passive regeneration and a higher SCR efficiency is possible than with the compromise, however the NO2 amount into DPF is still dependent upon the needed NO2/NOX ratio into the SCR.
As explained above, there is a problem associated with prior art methods and—apparatuses for purifying exhaust gases from a diesel engine.
The object of the invention is to provide an exhaust gas after treatment system and method which at least reduce the above mentioned problems.
The objects are achieved by the features of the independent claims. The other claims and the description disclose advantageous example embodiments of the invention.
According to a first aspect of the invention an exhaust gas after treatment system comprising a diesel particulate filter unit (DPFU) arranged downstream of a diesel engine. A NO2 reduction catalyst arranged downstream of said DPFU. A selective catalytic reduction (SCR) arrangement arranged downstream of said NO2 reduction catalyst. A first injector for feeding reducing agent into the exhaust gas arranged downstream said particulate filter and upstream said NO2 reduction catalyst. A second injector for feeding reducing agent into the exhaust gas arranged downstream said NO2 reduction catalyst and upstream said selective catalytic reduction (SCR) arrangement.
An advantage with the present invention is that it allows high noble metal loading on the DOC and/or the DPF resulting in good NO- and HC-oxidation and at the same time as an optimal NO2/NO ratio may be achieved for the SCR.
Another advantage with the present invention is that one may achieve high passive regeneration and HC oxidation in the DPF system and maintaining a good NO2/NO ratio for high NOx-conversion in the SCR-system for a fresh as well as an aged system.
Another advantage with the present invention is that it will be possible to use a smaller SCR-catalyst, giving both cost, space and weight benefits.
In yet another example embodiment of the present invention said DPF is coated with an oxidation catalyst material. An advantage with this embodiment is that one will still further save space, cost and weight.
The DPFU may have the DOC upstream of the DPF.
In still another example embodiment a heat generator is provided between said internal combustion engine and the DPF or DPFU. An advantage with this embodiment is that one may control the temperature in the EATS in an appropriate manner, for instance to suppress or activate one or more of the reactions taking place in the EATS.
The heat generator may consist of a DOC able to convert fuel to carbon dioxide and water. The heat generator may consist of a burner. A NO2 sensor may be provided downstream and/or downstream the NO2 reduction catalyst. The NO2 sensor may be a virtual sensor. The NO2 reduction catalyst may be applied as a zone coating on the DPF and said first injector may be arranged upstream said zone coating. The NO2 reduction catalyst may be arranged as a zone coating on the SCR arrangement and said first and second injector may be provided as a single unit arranged downstream said DPFU and upstream of said NO2 reduction catalyst and SCR arrangement.
According to another aspect of the present invention an exhaust gas after treatment method comprising the actions of: oxidizing NO into NO2 and trapping combustion particles in a diesel particulate filter unit (DPFU) arranged in direct fluid communication with an internal combustion engine, regulating NO2 content in a NO2 reduction catalyst arranged downstream of said (DPFU), reducing NO2 into NO in a selective catalytic reduction (SCR) arrangement arranged downstream of said NO2 reduction catalyst, injecting a reducing agent by a first injector into the exhaust gas arranged downstream said DPFU and upstream said NO2 reduction catalyst, injecting a reducing agent by a second injector into the exhaust gas arranged downstream said NO2 reduction catalyst and upstream said selective catalytic reduction SCR arrangement.
According to another aspect of the invention a computer program storable on a computer readable medium, comprising a program code for use in a method comprising at least the steps of the exhaust gas after treatment method.
This computer program can be adapted to be downloaded to a support unit or one of its components when run on a computer which is connected to the internet.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.